1. Field of the Invention
This invention relates to strong, water-insoluble, porous siliceous fibers, and to the process for preparing them.
2. Description of the Prior Art
In J. Amer. Ceram. Soc., 27, 299-305 (1944) Nordberg shows the preparation of 96% silica glass by heat-treating preformed bulky alkali-borosilicate glass objects above the annealing point, but below the deformation temperature, whereby the glass separates into two phases. The unstable phase, rich in boric oxide, is leached with water or acid to yield porous highly siliceous objects (reported to have surface areas of about 200 m.sup.2 /g and average pore sizes of about 20 A). The porosity is then removed by heating at about 1200.degree. C. Fibers are not mentioned.
In U.S. Pat. No. 2,461,841 Nordberg shows preparation of porous, highly siliceous fibers, not more than 0.001 inch in diameter, by leaching melt-spun alkali silicate fibers with aqueous solvents of pH not greater than 7. No specific degree of porosity is disclosed.
In U.S. Pat. No. 2,843,461 Labino shows leaching of flame-blown alkali silicate fibers less than 1.5 microns in diameter with aqueous ammonium chloride to yield porous fibers of almost pure silica. No specific porosity is disclosed.
In U.S. Pat. No. 3,110,545 Beasley et al. show rapid drying of thin films of aqueous sols of various oxides, including silica and alumina, in an edgewise manner whereby the dried film fractures into porous filaments of rectangular cross section.
In U.S. Pat. No. 3,681,017 Butcher et al. teach the preparation of porous silica platelets having surface areas of 400 to 500 m.sup.2 /g and an average pore size of 70 A by freezing and then thawing an ammonium stabilized silicic acid solution. The solution may have a pH from 2.5 to 10.5. The solution may be stored from 10 minutes to weeks or more before freezing. Freezing is carried out in the range from -5.degree. to -195.degree. C.